The hardness and wear resistance of a sliding member, such as a piston ring, are typically improved by coating the base member with a hard carbon coating, such as a diamond-like carbon (DLC) coating.
One of the objectives of patent literature (PTL) 1 is to provide a sliding member that has a hard DLC coating with excellent wear resistance. PTL 1 discloses a sliding member that has a base member, an intermediate layer formed on the base member, and a DLC coating formed on the intermediate layer by ion plating. PTL 1 discloses that the indentation hardness of the coating is preferably 20 GPa to 70 GPa.
One of the objectives of PTL 2 is to make high durability compatible with a low coefficient of friction. PTL 2 discloses a sliding member that has a base member, a lower layer coating made of DLC and formed on the base member by ion plating, and an upper layer coating made of DLC and formed on the lower layer coating by CVD. The indentation hardness of the lower layer coating is 20 GPa or more and 45 GPa or less, and the indentation hardness of the upper layer coating is 5 GPa or more and 20 GPa or less.